1. Field of the Invention
This invention relates to the shaping of glass to a desired form. It has for object to make it possible to obtain a high and accurate surface finish in a relatively simple and economical manner.
One application of the invention is to the shaping of glass articles, such as lens blanks, by a moulding or pressing process, with the object of producing on the article at least one surface which, on completion of the glass shaping process, will need relatively little or no finishing treatment to be of substantially optical quality. Another application is to the shaping of flat or ribbon glass by means of rollers, with the object of producing a ribbon having at least one surface of high quality, requiring little or no finishing treatment.
2. Description of the Prior Art
Glass articles to be used in the formation of optical equipment such as lenses and prisms are at present formed by processes which involve the formation of a blank which is subsequently ground and polished, or re-pressed from fire finished glass rod in highly polished moulds to give the final accurate surface of optical quality. Grinding and polishing is an expensive and time-consuming process, and a reduction in the quantity of glass to be removed from the blank to form the final article can result in a substantial increase in throughput where large numbers of blanks are being processed. Re-pressing is virtually a one-off, hand-operated process, involving re-heating fire polished glass rod and pressing in highly polished moulds. Re-pressing is used where polishing and grinding are unusually difficult and expensive, e.g. in the formation of aspheric lenses of the "Bulls Eye" type. Another problem occurs with glasses which are very fluid at the liquidus temperature, since they pour too freely from the supply of molten glass, which must be held above the liquidus to avoid risk of devitrification. In the case of lens blanks, this problem has usually been dealt with by using a technique known as "puddle loading", in which the glass is allowed to pour directly into the mould, as it will not form a gob which can be cut off and allowed to drop into the mould as in conventional lens moulding processes. The blank produced by puddle loading requires considerable grinding and polishing to achieve a finished state. Optical glass is also continuously cast into blocks for sale. In this form the end user cuts the glass into convenient sections for re-pressing. It is not possible to cast optical glass in relatively thin sections, e.g. of the order of 4 mm thick and 60 mm wide, because contact with solid mould surfaces during casting causes surface damage and ripples which entail polishing and grinding the surface of the blocks, and this would not be economic for the smaller quantity of glass in thin sections.
In the moulding of, for example, lens blanks, the conventional process involves the use of a rotating table provided with a series of moulds which are indexed round, being fed with a glass gob or puddle-loaded at one station, the glass being pressed at another station, and taken out and transferred to a lehr at the last station. The sequence of operations can be carried out rapidly because no two operations are physically carried out at the same position on the table. Precise temperature control of the individual moulds is, however, difficult to achieve without measures which would be prohibitively expensive for a product such as lens blanks, though individual mould temperature control systems have been suggested for use in forming more expensive articles, such as face panels for television tubes.
It will be evident that the processing time from glass feed to finished shape would be reduced if the process of surface finishing could be eliminated or if such an improvement in the finish of the article or shape when it leaves the shaping process could be effected that the amount of finishing (e.g. grinding or polishing) required was considerably reduced.
It has long been known that hot glass which can distort or flow, e.g. which is in a softened or even liquid condition, can be supported on a film of air without contacting the supporting surface. Thus sticking or damage to the glass surface would, it was realised, be avoided by providing such a cushion support. The only really large scale application of this technique has been in the processes used for bending glass in sheet form without marring the surface, in which the hot, softened glass sheets are supported on and fed along a contoured bed through which air is blown through a large number of small apertures, while the sheets sag into the desired curved form. Suggestions have also been made for feeding molten glass between porous rollers and then supporting the glass sheet formed on a porous air bed. Contact of glass with the rollers, it was thought, would be prevented by a cushion of air or other gas being supplied through the pores in the rollers. Another technique involving the use of a gas cushion has been employed in the blow moulding of electric lamp glass envelopes in paste moulds, in which the paste is moistened before moulding so that a cushion of steam forms between the mould surface and the glass.
The previous proposals for the use of a gas cushion support were thus directed to reducing or preventing contact between the shaping surface or surfaces and the glass being shaped during shaping or subsequent processing.